"Sri Lanka's President Maithripala Sirisena must immediately halt his plans to resume execution of at least 13 prisoners convicted of drug-related crimes," Amnesty International said in a statement.
According to local media, executions could start during national drug eradication week (June 22-July 1).
"Several sources have confirmed to Amnesty International that the newly recruited executioners have been trained and that President Sirisena has every intention to resume hangings," the NGO said, although there remained no official confirmation, Efe news reported.
Amnesty's South Asia Director Biraj Patnaik said, "The last thing that Sri Lanka needs right now is more death in the name of vengeance."
The Amnesty said lack of transparency on the part of the government "is preventing the public and human rights activists from accessing vital information to ensure that critical safeguards put in place by the international community to protect the rights of those facing the death penalty are fully observed".
The death penalty remains in force in Sri Lanka for crimes, including treason, murder and drug-trafficking, although the last case that received the presidential approval necessary for its application was in 1976.
At the start of the year, prison authorities received 102 applications for the job of executioner, vacant for years. Sirisena devised an anti-drugs plan based on practices in Singapore and the Philippines, the latter among the deadliest in history.
It was not the first time Sri Lankan authorities have tried to find executioners. But on several occasions, the candidates selected left before even completing training.
( With inputs from IANS )